Multi-threshold CMOS (MTCMOS) circuits facilitate low-power operation of many modern devices, particularly portable battery powered electronic products. Low-power operation is sometimes also referred to as standby or sleep-mode operation in which certain non-critical circuitry is disconnected from power and/or ground connections, with supervisory circuitry remaining powered for data retention and for reestablishment of active mode operation upon detection of certain operating conditions and/or after a predetermined period of time. For example, mobile phones not currently supporting a call session may enter the low-power operating mode and periodically “wake up” for communications with a base station, and if no call is directed to the phone, the device may resume low-power operation in order to conserve battery power. In addition, many applications require retention of data during low-power or standby operation. High speed operation and active mode efficiency are facilitated by use of low voltage CMOS circuitry with low threshold voltage (low-Vt) transistors. Accordingly, many digital circuits are designed around a standard transistor threshold voltage (SVT) which is relatively low in order to enhance efficiency during active mode operation, and some circuits may include even lower threshold voltage devices (LVT). However, the low threshold voltage of such devices may lead to unacceptably high levels of leakage current during standby operation. Multi-threshold CMOS circuits employ power disconnection transistors with higher threshold voltages (HVT) to disconnect power and/or ground connections from the lower threshold voltage devices, and to construct balloon or shadow latch circuits for retaining data during power down of the remaining circuitry. However, conventional HVT-based retention flip-flops and other sequential circuits suffer from poor performance and lack of robustness, particularly at low operating voltage levels. Conversely, LVT or SVT sequential circuits suffer from high leakage in the low-power retention mode. Accordingly, a need remains for improved MTCMOS sequential circuits providing the capability for low-power retention mode with low leakage currents, while providing high-speed active mode operation for ultra-low-power and other applications in which power efficiency is important.